text:
Could you use radial blowers to propel a model aircraft?
I was inspired by stealth planes like the Boeing Phantom Ray:

I'd like to make a model aircraft that's as thin as possible, and has integral engines that are not visible from the outside. I have been wondering how this might be possible.
One way might be to place a radial blower in the centre of the fuselage. Radial blowers suck in air through their rotary axis, and blow it out the sides.

Usually, they are housed so that air is constrained to come out in one direction. However, you can always give the air two exit ports.
I wonder if something like this might make a good engine for a plane? The air intake would be at the top of the plane, and the exhausts would be at the back.

Questions:

Would a single large engine have a power:weight benefit over two smaller engines?
Would the intake actually provide additional lift by reducing the pressure above the plane?
Might be intake be starved of air if the plane is acting as a lifting body?

Without knowing the actual physics, I don't believe that two port system would work, the thrust would be terribly uneven.

@CGCampbell - Why do you think that would be? Would it still be uneven if I made the ports symmetrical?  How about if I added throttles to each port, and servoed them to enforce symmetrical thrust?

The bend in the ducting would rob the one on the right side of a fair bit of thrust. I'm not even sure if much of the airflow would wind up reaching that duct, as the left side gets ~1/2 of the fan while the right gets about 1/6.

@egid that is what I was seeing, the left gets the natural airflow, while the right would be flowing forward and be ducted rearward...like I said, I don't know enough physics to understand WHY it wouldn't work (be efficient enough) but I have a strong hunch I'm right. I'll wager Mr. Kampf will chime in before long.

Interesting question, and I can only answer in general terms because much depends on speculation.
Short answer:

use one central exhaust
one big blower is better than two small ones
one short flow channel is better than two long ones
suction on top might be beneficial for high lift

Radial-flow compressors are very light and achieve high pressure gains per stage. However, the simple plastic thing in your illustration will not have good efficiency. Generally, cambered blades are better than straight blades, and the thinner they are the better. Thick blades block more of the flow path. The drawback is their high diameter, and by placing the blower on its side, you get around this obstacle in an elegant way.
The idea of using two exhausts complicates things; if you use a single one, you can avoid problems due to unsymmetrical thrust. Given that the compressor runs in one of two directions, asymmetries will be very hard to avoid. By placing vanes into the exhaust you can actually give the model plane some degree of sideways thrust vectoring. Also, two flow channels will have more surface area per cross section than a single channel. Your losses will be higher if you use two like in the illustration because they are longer than necessary.
The same goes for the blower: A single, larger one will always be more efficient. The gap size in a small blower is probably relatively bigger than that of a larger blower, and gaps are real efficiency killers.
Another efficiency killer is flow separation in the ducts. Make sure that cross section changes are gradual and avoid bending the flow as much as possible. It will probably help if you install cambered guide vanes at the entry of the exhaust to convince the rotational flow to change direction and leave the round compressor chamber.
Suction on top is also interesting. There have been many ideas about boundary layer suction to increase maximum lift, and your turbine inlet will certainly help to remove some of the boundary layer where it will separate first. Normally, turbine intakes are placed where the normal flow field already shows some pressure increase due to ram effects, but at the speed of a model airplane this will not be a factor. I expect the best intake in your case is a hole on top of the aircraft with nicely rounded edges. Those rounded edges should already produce some lift, and the whole configuration will probably achieve a higher stall angle of attack when the engine runs at full power than at idle power.
I can only speculate how much power will be converted to thrust, but I bet placing an ordinary propeller at the usual place will be more efficient. But the configuration you propose has its own merits.

I agree with @PeterKampf, you want a single engine with thrust applied down the centerline of the model, it will be very difficult to balance split thrust whether it's one engine or two.

I have to add that a single rotating disk would cause a yaw tendency in the other direction.

@SMSvonderTann: Only if the airflow isn't straightened on the way out. If there is no asymmetry in both the inflow and the outflow, how can there be a yaw tendency? The blower torque is counteracted by the force on the exit vanes.

@SMSvonderTann There will be some yaw due to motor shaft torque, it should be relatively constant and might be possible to cancel with outlet geometry.

Some smaller turbine engines actually do use centrifugal compressors for various reasons. In larger turbines they become impractical, but that wouldn't affect this application.
The centrifugal fan causes the air flow to turn 90 degrees. In computers this can be good, mounted above a heat sink, where they draw air through the heat sink, and then sideways out the exhaust (or sometimes vice versa). Efficiency is not as important. In your application, the aim of the propulsion system is to increase the air velocity versus the air moving past the plane. The fan is making the air turn 90 degrees to come into the fan intake, and then 90 degrees back to exit the exhaust.  The turns in the air flow will introduce losses in the flow, and it's much easier to make smooth flow in and out of an axial fan than a centrifugal fan in this case. A centrifugal fan can also be a heavier installation than an axial fan.
In general, one fan will probably be more efficient than two, because you would be adding weight for the installation parts, wiring, etc., of the second engine, and not gaining much unless you really need the power of a second engine, or have some redundancy/configuration requirements.
I think an intake on top of the fuselage would hurt performance more than help it. By disrupting the air flow, it would probably cause more reduction in lift and increase in drag than the low pressure intake would benefit it. Having an intake more like the one in the above picture that just feeds a centrifugal fan rather than an axial one would probably be a better option.
Having one exhaust will probably be a more simple design. Guide vanes or even a vectored exhaust steer the exhaust better if needed.
Aside from the issues in the integration:
Here is a comparison of various parameters for axial vs. centrifugal fans. The backward-inclined flat blade configuration shows a graph that is actually relatively similar to the axial fans, aside from the power curve. Als, here are a couple interesting summaries of axial fans and centrifugal fans. They have a more industrial focus, but still have some relevance.

The definitive answer is YES, as demonstrated by this guy who built one:

That is so cool, no danger of getting hit by the prop.

One could mount the impeller around the motor for a more compact look and have dual sided inlet and impeller for slightly increased efficiency (maybe).


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Are there any airports that do not have an IATA code but have a huge number of traffic?
I am collecting a list of all airports in the world for my project, at present I have 46000 airports in my database but only 7000 airports have IATA code. So my question is, if I only take 7000 airports with IATA code and delete the rest will I lose any major airports or airports with huge traffic.
Are there any airports that have no IATA code but do have a huge passenger traffic or provide connections to major airports?

If you are collecting all the airports in the world, why do you want to eliminate a large chunk?

I only want the airports with passenger traffic. The database is for creating a flight booking site. So there is no use in having airports which passengers never use (i,e) Why i planned to delete all the airports which doesn't have an IATA code. Before deleting i wanted to confirm are there any airports which has no IATA code but provdes a major connection or serves a huge passenger traffic.

You mean commercial traffic. Passenger traffic is at many airports that don't have IATA codes.

Can you name a few airports Burhan.It will be very helpful for me.

@BurhanKhalid I assumed he meant scheduled passenger airline traffic, which is a subset of commercial traffic. Lots of commercial traffic is non-passenger and operates out of fields without IATA codes (e.g. cargo, crop dusters, aerial photography, air ambulances, etc.)

Technically, Groom Lake has a pretty substantial (but almost certainly classified) amount of scheduled commercial passenger traffic, but no IATA code. It's a popular destination for JANET airline, which may also be the only airline whose flight attendants are required to acquire and maintain a U.S. DoD Top Secret clearance. :) I don't think they have any interlining agreements, though, so you probably don't need to worry about them in your booking system.

According to this thread, pretty much anything with scheduled air service will have an IATA code to facilitate booking.
From looking at the FAA data, it looks like everything with more than 10,000 enplanements (enplanement=1 passenger boards an aircraft) is considered a "primary" airport, and they all have an IATA code unless the airport is strictly general aviation and unscheduled service.
For these reasons, you probably won't find anything with "huge" passenger traffic but no IATA code. There may be airports with unscheduled service to major hubs, though.

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text:
What would happen if someone covered a small hole in the fuselage of a plane with a soft body part?
If I punched a hole of the size of a small coin into the fuselage of a plane, and then covered it with the palm of my hand, what would it happen to my hand?

I think the answer depends on when you punch that hole

You'd be better off using a large coin to cover a hole the size of a small coin ;-)

You may ask xkcd

That's actually a good idea

I assume the hole has been properly deburred, otherwise you could suffer cuts on the sharp edges of the hole.
What happens else depends on altitude and flight speed.
At low level, the outside temperature should be tolerable and the pressure difference between inside and outside should be small, so your hand will be fine.
At higher altitude, outside air temperature can drop to -60°C rsp. -76°F. Since air density is lower, the aircraft will travel fast (approx. 250 m/s rsp. 560 Mph in case of an airliner), and convective cooling at the hole will soon cause hypothermia and frost bites. Also, inside pressure is 0.6 bar rsp 8.8 lb/sq-in higher than outside pressure, and the exposed skin on your palm will feel this as a sucking sensation.
I further assume that you need to remove all cabin insulation to put your hand over that hole, so your whole hand will be exposed to the airplane's metal skin which will have the same temperature as outside air. If the hand is dry and does not freeze to the metal skin, you might manage to keep it in place for a few seconds, but it will hurt. Keep it in contact with the metal skin longer and you will suffer progressive frostbite. The part of the skin which covers the hole should suffer less, since convective cooling should be less intense than the thermal conductivity of the metal skin, which will quickly pull all body heat from your hand where it touches the metal.
The pressure difference will not damage your hand, but the low temperature will.

Great! I didn't expect the temperature to be the biggest problem.

@Xocoatzin What did you expect to be the biggest problem?

@Simon Probably having a much bigger pressure difference that would crunch the bones of the hand and extrude your flesh out of the plane.

@Xocoatzin The pressure itself is not much a problem. Remember that during a decompression at 10km, you'll happily survive, given you put on the mask. Some people may feel sick, but that's it.

Would plugging a cork do the trick..

@Xocoatzin Pressure doesn't work like that.  Think about someone on the summit of Everest.  There is no negative affect from the reduced pressure, only from the reduced partial pressure of oxygen.  Bodies exploding in a vacuum are just the stuff of Hollywood.

@yo' simon: The low pressure itself, no, but, if the decompression is fast enough, it can easily cause burst eardrums and/or lungs (that's why it's called explosive decompression).

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Would an airliner be allowed to take off in snow?
Im going to fly to Tucson tommorow. The weather is telling me its going to snow in Denver. So would snow cause a delay or are airliners allowed to take off in this kind of weather?

I think the snow amounts are less than an inch

The options "allowed to take off" and "would cause delay" are not mutually exclusive.

So long as you are at an airport where snow happens it won't be a problem.  Locally, less than 2" shut the airport down because that just doesn't happen--no de-icing equipment so nobody could take off.

I lived/worked in Juneau. AK, for 6 yrs with regular trips to Seattle and south. Fun coming in in December/January snow storms with heavy winds, but never a delay except slower flights due to headwinds. Pilots were competent due to lots of practice.

Don't worry about it
While snow can sometimes cause a delay, airports are really good at snow removal.  (Especially airports like Denver that are used to getting a lot of snow.)
According to flydenver.com DIA has 500 trained snow removal people and 270 pieces of equipment to clear 12.2 million square feet of runway, taxiways, etc.
To put that in perspective, a typical highway (2 lanes each way, with shoulders) is around 38 feet wide each way, or 76 feet for both directions.  If I've done the math correctly, 12.2 million square feet is about 30 miles of road.
The state of Minnesota knows a thing or two about plowing snow.  They use about 1500 drivers and 839 plows to handle 12,000 miles of road.  To plow snow the way the Denver airport does it, they would need 200,000 drivers and 108,000 plows.
Here's a YouTube link that shows off some of Denver's bigger snow removal tools.
Chances are, if you can get to the airport you'll be able to fly (but perhaps not on time)

Some airports are terrible at getting rid of snow, however.... Heathrow often comes to a standstill after a light dusting

Yes
There are two considerations in snow:

Icing
Visibility

Visibility is easily dealt with and most airlines have approved opspecs to takeoff in low visibility (e.g. down to 300 or 500 feet RVR).  Heavy snow can reduce visibilities to less than a quarter mile (e.g. less than 1200 feet RVR) so there is quite a bit of margin between between typical snowfall and minimum visibility takeoffs.
Icing is a bit more of a challenge, but is a well defined problem with specific procedures.  In snow airplanes will be de-iced with Type I fluid and then treated with an application of Type IV fluid for anti-ice protection.  This fluid has a holdover time that you can takeoff within without additional checks.  Some of these holdover times can be low (e.g. 5 minutes for specific conditions) but de-ice is usually where the takeoff bottleneck is and once you are treated you can generally take off without too much further delay.
Planes will be able to operate in snow, but departure rates will be less than they would be for a normal good weather day.  Departure rates will be limited by runway condition (the airports ability to keep them treated and clean), the throughput of the de-ice facilities and to some degree the aircraft arrival rate (you don't have planes to fly out unless planes are making it in).
Winter ops are normal this time of year (northern hemisphere).

Continuous heavy snowfall can eventually close an airport if it becomes impossible to keep the runways clean, taxi routes become unusable due to drifting snow or snow transitions into sustained freezing rain (or runs out of de-ice fluid, yes IAH I'm looking at you).  This is a transient problem and the closure would end as soon as conditions ease and the facilities can keep up with it.

This answer does make me wonder if there are other ways snow can foul up an airport...perhaps by interfering with the ILS or lighting systems?

@UnrecognizedFallingObject That deserves to be a question rather than a comment.

Yes, heavy snow delays departures from the Denver airport (based on personal experience). Of course, nothing  horrible: the airport will be cleaned, the plane will be de-iced (twice if your are unlucky enough), and may even catch part of the lost time on the way to destination. Still, it would be unwise to rely heavily on very on-time arrival.
If you need to change into another flight at your destination, and especially if you need to do anything else on time where the airline cannot help (change into unrelated train, or attend important meeting), I would advice to add a few extra hours for the possible delay.

I flew a few times out of some very cold places... on video shoots (I work in film/video). Places like Albany NY, Buffalo, and Wyoming.
Those airports are pre-pared to deal with the snow, so what would fully shut down an airport like say Austin; its business as usual in Albany.
Being in a window seat, and watching a guy in a cherry picker with a fire hose outside your plane spraying down the entire aircraft with I assume a de-freezing chemical solution similar to whats in your windsheild wiper spray; was pretty scary the first time. It was snowing hard. Everything was covered.
But they sprayed us down; and off we went; in a 777 no less.

You flew out of Albany, Buffalo, or Wyoming in a 777?!


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How long does it take to restart a turbofan engine inflight?
I'm especially interested how long it would take to restart a turbofan engine after a hypothetically erroneous shutdown of an engine immediately after takeoff i.e., at an altitude at which the air is reasonably dense. Is windmilling enough or would you need crossbleed and starters again?

It can take from a few seconds to eternity. If the core cools down, the engine might lock up.
Windmilling starts are only possible if the aircraft is flying fast and low enough. It will assist the startup process, but needs additional help from the starter or bleed air, typically below 300 knots and above 20,000 ft. Details vary from type to type. If everything works as designed, a windmilling assisted restart is faster than a regular engine start.
The ability to restart in flight must be proven as part of the certification process. However, right after takeoff the speed is too low for a pure windmilling start, and an additional energy source is needed.

So it seems that for most/all commercial aircraft, starter or bleed air would be necessary, since the question was a hypothetical shut down immediately after take off, and the Concorde was the only (one of the exceptionally few?) commercial aircraft to take off at over 200 knots.

@FreeMan: Right, I added a sentence which confirms this. I would expect that the Cocorde needed even more speed for starting the Olympus 593 by windmilling. Same goes for the Tu-144 and its Kusnetzovs, btw.

@jjack That's a good question. Why don't you go ahead and ask a new one.

@PeterKämpf: Actually, it would probably need less speed for a windmill restart than the engines on most jetliners, since the Olympus 593 is a turbojet (where all the air entering the engine passes through the core - that being the entire engine for a turbojet - and helps to spin the compressor and turbine rotors), whereas modern subsonic jetliners use high-bypass turbofans (where at most 20% of the air coming in passes through the core, and the remaining 80%+ passes uselessly through the bypass duct).

@Sean:  Those 80% are not useless - after all, the compressor is coupled to the low pressure turbine, and spinning this up increases the mass flow there so the high pressure parts can spin up as well.

Right after takeoff bleed air would be necessary to restart, preferably from the APU if thrust reduction on the operating engine is unacceptable in the current phase of flight.
I only have data for the Rolls Royce Allison AE3007 A series engines and the windmilling envelope at all altitudes is between 260 KIAS and Vmo/Mmo with the additional limitation that N2 must be above 10%.  An aircraft that has just taken off will not be going this fast so an assisted (bleed air) start is the only viable option.
How long the start will take depends on the particular engine, but all turbines (that I'm aware of) have a maximum start time limitation.  The engine discussed above on the EMB-145 has a 60 second limitation on starting.  Based upon my experience starting the engines with a stiff headwind, I'd assume that an APU start at flying speeds would probably take around 35-40 seconds in the EMB-145.  By the book the engine needs another minute before significant thrust is applied.
All of the above information is specific to one airframe/engine combination and will vary for other aircraft and engines.


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What is the cable connected to a plane for?
I've seen that airplanes are connected to some wire like in the picture. What is it's name and what is it for?

This is a ground power unit (GPU) which supplies the aircraft with electricity while the generators or the auxiliary power unit (APU) are not running. This is important especially during boarding, when the cabin lighting needs to remain on for passengers to embark or disembark. The GPU is also used to start the APU, which in turn provides electricity to start the engines and the generators.

A ground power unit (GPU) is a vehicle capable of supplying power to aircraft parked on the ground. Ground power units may also be built into the jetway, making it even easier to supply electrical power to aircraft. Many aircraft require 28 V of direct current and 115 V 400 Hz of alternating current. The electric energy is carried from a generator to a connection on the aircraft via 3 phase 4-wire insulated cable capable of handling 261 amps (90 kVA). These connectors are standard for all aircraft, as defined in ISO 6858.
  Source: Wikipedia

Related Questions:
Can large airliners be operated without ground support?
How are turbine engines started?

Nevertheless aircraft often have APU running during boarding, because while GPU provides electricity, it does not provide pressurized air for air-conditioning.

@JanHudec - more often than not, the AC is NOT running during boarding, and the airline apologizes that they can't turn it on until the doors are closed. At least that's been my experience in the US.

I am surprised by the use of 400 Hz for the AC. Why such high frequency?

@Floris Higher frequency generally means smaller, lighter transformers.

@Geobits - that makes sense. Thanks!

@JanHudec It is fairly common to use a source of conditioned air from the ground as well so that the APU isn't running when not necessary.

that doesn't look like a GPU, which is a generator on wheels, like so: http://www.weyer.aero/ground-power-units.html . Its just an electrical power hookup, and I don't think falls within the category of GPU

@rbp The term GPU does not only refer to generators with wheels. It is also used for mounted systems on jetways and systems let into the surface of the airport. "Some airports, like Tokyo-Haneda, have ground power feeds built right into the surface of the apron." Ref: http://www.airlinereporter.com/2014/07/ramp-operations-whats-going-on-out-there/

@Floris My experience in the U.S. is that A/C is usually running while parked at the gate, but that the A/C is sourced from an A/C unit mounted on the bottom of the jetway and connected to the aircraft with a large air hose. Some of these systems don't seem to be as powerful as the A/C provided by the aircraft A/C packs in normal operation, though. The presence and effectiveness of these systems can vary from one airport to the next, though.

@reirab thanks. I have sat in many a hot plane with the announcement that AC won't start until the engines do. In the US. On some very hot days. This is clearly not universal. Unfortunately.

@Floris Yes, as I said, it varies from airport to airport. Also, it's possible in some cases that the FAs mean that the aircraft's (significantly more powerful) onboard A/C system can't be started until the door is closed. Especially on particularly hot/sunny days, the external A/C sometimes isn't enough to keep up with the heat, especially with hot air coming in through the open boarding door.

It provides power to the plane for when the engines and APU are not running while sitting at the gate to save fuel.
It allows the cabin lights to remain on so the passengers aren't tripping over each other as they board.
The power may also be used to start the APU that will in turn provide the power needed to start the main engines.

There are two kinds of GPUs:

The ones that are fixed and supply power to the aircraft
The ones which are mobile, which are tugged to the stationary aircraft

The cables are connected by groundcrew before the pilot shuts down all engines. It is different from pilot to pilot. The APU remains on while the aircraft is being serviced.

The APU remains on. Cargo doors are opened . FMC / belt loader start unloading cargo from airplane. The gbu remains on thoughtout unloading and loading of aircraft.In certain cases the gpu might trip for certain unknown reasons

GPU is the cable you are talking about in your photo. This is called GPU ground fixed unit at a operating aerodrome.

Hello RAYHAN ahmed, welcome to Aviation.SE! To edit your post and add information to it you can use the edit link under your answer.

Gbu means GPU my spelling mistake sorry.

@RAYHANahmed Can you please update About Me section describing what you do. If you just reference about it in an answer, not everyone will understand as they may not know about your other question, where you described it.

You will find my question to put to a GPU expert (google) my electrical engineering.

Concerning a PAkistan airlines 777 that we were offloading  with non operational APU and a tripping GPU.

I work as a cargo aircraft loader with wide experience loading cargo pallets courier human remains. Animals etc on heavy transport aircraft since July 2002 to date.

To farhan : google the following  - my electrical engineering  by Steve McFadyen  ( what is aircraft ground power)

Then look under ; my electrical. By Steve mafadyen

It is ground power unit.
Due to the technical difficulty on ground to operate APU, which is the source of power for the aircraft.
Aircraft engineers connect GPU.

technical difficulty on ground to operate APU? could you expand on this?


text:
How to find polar curves of airliners?
The pilots operating manual of our Cessna 182 contains the lift-versus-alpha and lift-versus-drag data of the airplane, so I assume this is data that should be publicly available.
Is there a trustworthy resource where I can get the polar curves of different aircraft, especially airliners? I'm looking for that of the Airbus A350 XWB in particular.

Polar curves for older airliners can be found in lecture notes or technical publications. The drag polars of the most recent ones, where no independent measurement is yet possible, are a closely guarded secret. If you see something, it will be restricted such that an important part of the puzzle is missing. For the polars to be meaningful, you need aircraft mass and flight conditions, air temperature and density and sometimes also engine thrust information to make any meaningful comparisons. Also be aware that the Airbus method to determine the reference area is different from Boeing's, but at least those methods are published. Full disclosure is only available to potential customers on the basis of an Non-Disclosure Agreement (NDA).

where no independent measurement was yet possible, you mean is?

Yes, I wanted to express that no A-350 is in operation and available for independent evaluation. Yet.

I know of no source for the A350.
I was moderately surprised to learn, however, that there are surprisingly well-modelled drag curves for many modern jet transport aircraft including the Boeing 777 in a book called Aerodynamic Design of Transport Aircraft by Obert. They will in all probability not be 100% accurate, but they work much better than required for your run-of-the-mill back-of-a-napkin calculation. Keep in mind that for any meaningful analysis, you need a fairly accurate engine model, too, and before going into even more detail you need to be sure whether engine nacelle drag is included in the drag curve or the engine model or (worse!!) in both or neither.
Also, if you just need some independent data to innocently play with, google "Piano X 787 sample analysis".

You may choose to download this Excel spreadsheet which contains a list of over 100 glider performance figures in the tab "Gliders" . The spreadsheet allows comparison  of two aircraft **Polar Curves*.
Boeing and Airbus does not make public the figures pertaining to Drag Polar Rates.
You can find some assumptions here.


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Why is the Airbus A320 permitted to carry 180 passengers, instead of the maximum 179 allowed?
The EASA regulations give the following requirement for number of emergency exits:

If then the Airbus A320 is certified to carry 180 passengers, it seems to me that it falls just outside the minimum bracket of 2x Type I exits and 2x Type III exits and it should then have a pair of additional Type III exits to be able to accommodate 180 passengers.
However, various sources give the emergency exits as (link):

This configuration, in my opinion only allows 179 passengers, not 180.
Is there some clause I am missing?

Your diagram has 132 seats, for which the A320 has more exits than your EASA table requires. It would be better to find an Airbus plan that shows 180 passenger seats.

I wanted to present a diagram showing the exits rather than the seating layout as this varies from airline to airline. If you do a study on SeatGuru you will see that all A320's (that I have seen) have the emergency exit arrangement as shown above, e.g. easyJet seats 180 pax with this layout (http://www.seatguru.com/airlines/EasyJet_Airlines/EasyJet_Airlines_Airbus_A320.php)

See Page 9 of TCDS A.064 ANNEX - Airbus A318, A319, A320, A321 - Special Conditions

TCDS A.064 ANNEX - Airbus A318, A319, A320, A321 - Special Conditions
EQUIVALENT SAFETY FINDING         E-2107: Passenger Extension to 180
APPLICABILITY:                    A320
REQUIREMENTS:                     JAR 25.807
ADVISORY MATERIAL:                N/A

Statement of Issue:
An application was made on September 30, 1992 to JAA for certification of maximum number of passengers of 180.
Paragraph 25.807(c)(1) of JAR specifies for passenger seating configuration, the number and
  types of Emergency Exits for each side of the fuselage.
  The maximum seating capacity allowable under JAR 25.807(c)(1) for A320 exit configuration is
  179 (2 Type I and 2 Type III).
However, the exits size for front and rear doors on the A320 is such (32x73 inches) that they
  are oversized Type I and therefore can be considered as a non standard exit arrangement.
The same exit size is being used on a derivative of the A320, for which Latin Square Tests
  have been conducted in order to establish an appropriate rating for that kind of exits. These
  tests have shown that, when associated with the performance of the slide installed at these
  exits, a rating of 55 passengers is appropriate.
It should also be noted that the full scale emergency evacuation demonstration of the A320
  with 179 passengers has demonstrated compliance with JAR 25.803(c) with sufficient margin
  to justify 180 passengers.
Under those conditions, a maximum capacity of 180 passengers is requested for the A320.
JAA Conclusion:
  Based upon the demonstrated higher rating for the floor level exits and the adequate margins
  for 179 passengers shown in the 90 seconds demonstration, and used as a basis to show
  compliance to JAR 25.803(c), the JAA accept 180 passengers as the maximum capacity for the
  A320 on the basis of an Equivalent Safety Finding.

The table you posted is no more valid as per CS-25 Amendment 12 (PDF).
Now each Type I allows for 45 passengers and each Type III allows for 35.

Type and  number  required.
The maximum number of passenger seats
permitted depends on the type and number of exits installed on each
side of the fuselage. Except as further restricted in sub
paragraphs(g)(1) through (g)(9) of this paragraph, the maximum number
of passenger seats permitted for each exit of a specific type
installed on each side of the fuselage is as follows:

TypeA  110
TypeB   75
TypeC   55
TypeI   45
TypeII  40
TypeIII 35
TypeIV   9

The subparagraphs that apply to the A320:

(6) For a passenger seating configuration of more than 110 seats, the
emergency exits on each side of the fuselage must include at least two
Type I or larger exits.
(7) The combined maximum number of passenger
seats permitted for all Type III exits is 70, and the combined maximum
number of passenger seats permitted for two TypeIII exits on each side
of the fuselage that are separated by fewer than three passenger seat
rows is 65.

It looks like you only get to count the exits on one side of the aircraft to calculate capacity. The A320 only has 2 Type 1 on each side, which is 90; any number of Type IIIs add at most 70, for 160 max.

@cpast And the type III's aren't separated by 3 passenger seat rows, so they actually only count for 65, for a total of 155.


text:
Does acceleration increase linearly on a takeoff roll?
I'm a relative novice when it comes to aircraft performance and was wondering if someone could both answer the following and maybe direct me to some good source material.
If I wanted to model a take-off roll, can I assume a constant acceleration up until the wheels-up point?  If not constant, is there a reasonably good function that could model the velocity curve on takeoff?

Googling the title of your question found this paper which has time and distance calculations for the takeoff ground run, including the varying effect of thrust and drag. I have no idea how accurate or useful it is, though.

@Pondlife Thanks so much!  This is really helpful!

Are you referring to acceleration in one dimension (along the runway OR into the air) or in two dimensions (along the runway and into the air)?

@kjmccarx Just along the runway right up until the rotation point when the wheels leave the ground.

Thrust depends on speed and the type of engine. To simplify things, we can say that thrust changes over speed in proportion to the expression $v^{n_v}$ where $n_v$ is a constant which depends on engine type. Piston aircraft have constant power output, and thrust is inverse with speed over the speed range of acceptable propeller efficiencies, hence $n_v$ becomes -1 for piston aircraft. Turboprops make some use of ram pressure, so they profit a little from flying faster, but not much. Their $n_v$ is -0.8 to -0.6. Turbofans are better in utilizing ram pressure, and their $n_v$ is -0.5 to -0.2. The higher the bypass ratio, the more negative their $n_v$ becomes. Jets (think J-79 or even the old Jumo-004) have constant thrust over speed, at least in subsonic flow. Their $n_v$ is approximately 0. Positive values of $n_v$ can be found with ramjets - they develop more thrust the faster they move through the air.
Drag depends also on speed, and in addition on lift. During the takeoff roll the dynamic pressure grows with the square of speed, and drag is almost proportional to the dynamic pressure. Since the flow's Reynolds number also increases with speed, the zero-lift drag coefficient (aka friction plus pressure drag coefficient) decreases with speed. Depending on the ground attitude of the aircraft it will create already some lift during the rolling phase, but lift substantially increases during rotation when it is raised to lift the aircraft off the ground. When the aircraft accelerates further after liftoff, the lift-dependent part of drag goes down with speed while the zero-lift part continues to increase with dynamic pressure.
For most aircraft, thrust is highest when the aircraft is at rest (constant pitch propellers can have a lousy efficiency at takeoff when they are optimized for fast flight, so here you might have higher thrust at some positive speed), and decreases the faster the aircraft moves through air. Since drag is also lowest with the aircraft at rest, the highest acceleration is possible right after brake release. As soon as the aircraft rotates, the new lift-dependent drag component will cause a marked decrease in acceleration, and when the aircraft climbs, some of the excess thrust needs to go into climbing, so acceleration decreases again.
Newton's first law gives a formula for acceleration a: $$a = \frac{T - D}{m}$$
where T is thrust, D is drag and m is the mass of the aircraft. The integration of acceleration over time gives speed.
There is no single formula for the velocity curve, and my recommendation is to split the take-off into three sections: Ground roll, rotation and initial climb. In all phases you need to calculate with speed-depependent drag and thrust, so it will be best to integrate the parameters stepwise in small time steps.
The comments encouraged me to give a more detailed list of drag components. These here have to be considered during the take-off roll:

Zero-lift drag of the airframe (due to friction and pressure)
Induced (lift-related) drag for the aircraft in horizontal attitude, including reduction due to ground effect
Drag increment due to flaps in take-off position
Landing gear drag
Wheel friction (0.025 * weight on a hard runway, but a lot more on soft ground)
Kinetic energy loss or gain due to runway slope

With rotation onset these drag components must be added:

Trim drag due to elevator deflection to lift the nose
Induced drag at the actual pitch angle

When the aircraft lifts off, these changes must be considered:

Reduced power for acceleration due to power requirement for climb
No more wheel friction
When the gear is retracted, drag increases as the doors open and is greatly reduced once all wheels are stowed.
Ground effect is reduced as the aircraft climbs away from the ground

Don't forget to include wind speed in your calculation!


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What open databases with aircraft, airlines and country exist?
As the above question states, I'm looking for an open database that I can access. I'm building a plane spotter app, and need a lot of data.
The data needs to be downloaded.

Let's assemble our minions and build one! Haha. You might want to take a look at this: http://www.flightradar24.com/data/airplanes/
I'm not sure if they have an API for it though :)

That would be awesome! I kinda found somthing that I'm thinking about spidering. http://www.airfleets.net/recherche/country.htm Is it a good one? An API would not be enough.

I have spent the last couple of days to build a crawler for you, it is written in PHP, and is almost complete. For the moment: It stores all the data it crawls into a MySQL database table. If you want it stored im some other way: Let me know :)

Wow that's amazing! Currently working on it myself. I would love to take a look at it. MySQL database is perfect!

Add me on Skype so we can chat without filling up the comment section, I dont think the mods like it. My Skype is: djzorrow .
I have started to collect the data now, but with 23090 aircrafts: It takes quite some time.

Geographic (routes, land-based infrastructure) data:

http://ourairports.com/data/
http://openflights.org/data.html

Coordination (scheduling, flight numbers) data:

http://www.virtualradarserver.co.uk/FlightRoutes.aspx

Aircraft (registration) data:

List of national government registers - http://www.gatwickaviationsociety.org.uk/registers_official.asp
ICAO address country prefixes - http://www.kloth.net/radio/icao24alloc.php
(old) id and routing data - http://www.libhomeradar.org/download/index.php


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How is engine fuel kept from freezing?
The following table shows the operating temperature of Jet fuels (Source http://www.skybrary.aero/)
I know a lot of flights will be cancelled during bad weather conditions including the cold. How is the Engine fuel kept from freezing when the flight is operated at the mentioned fuel freezing temperature?

-40°C is very cold (-40°F in imperial units). You'll have different problems at that point.

@ratchetfreak the temperature at 10000m is well below that.

To expand on the above commenet, under the International Standard Atmosphere with a sea-level air temperature of 15°C, air temp at altitude can drop as low as -57°C.

@njzk2: Freezing points change with pressure also.

@anaximander It can get much colder at altitude than that.  I'm currently at 43,000 ft. and it is -61C.  However, this question appears to be asking about temperatures on the ground and there is another question which covers in flight fuel freezing.

@ratchetfreak Even for ground temps, -40 F/C actually isn't that unusual for parts of Alaska, Canada, Siberia, or even the Northern plains states in the U.S. during the winter. It is indeed ridiculously cold, but it's not that uncommon in some places (fortunately, where I live is not one of those places... -5 F is cold enough for me.)

There are multiple ways for preventing fuel freezing in aircraft.

In large aircraft, the fuel tanks have heating systems.
Fuels with low freezing point, like Jet A-1 can be used. In really cold conditions (like Canada, Alska, Russia etc) Jet B (with freezing point -60$^{\circ}$) can be used, though this has higher flammability. The military equivalent of Jet B is the JP-4.
An anti-freezing compound, called the the Fuel System Icing Inhibitor (FSII) is added to the fuel to prevent icing.

The problem in aircraft is not the fuel itself freezing, but the free water in aviation fuels freezing. All aviation fuels contain some (small) amount of water, which cannot be extracted. At (high) flight altitudes, the fuel cools and its capacity to retain dissolved water is reduced.
Some of the dissolved water separates out as discrete water that can form into ice crystals, blocking filters or form supercooled liquid which can crystallize and form ice crystals when it comes into contact with fuel lines.
FSII dissolves sparingly in fuel, but more easily in water. Any water present will extract FSII from the fuel; the additive then acts to reduce the freezing point of the free water, preventing the formation of solid ice crystals which could block the fuel supply system.
The FSII is usually ethylene glycol  monomethyl ether (EGME) or diethylene glycol monomethyl ether (DiEGME). These fuel icing inhibitor additives are mandatory in all military aircraft fuels (like JP8) and are optional in world-wide  commercial aviation fuels depending on route, flight length, and season.

Following your last sentence (and just out of curiosity), does it mean that the at the same time and at the same airport, two flights can get a Jet A with and without EGME, respectively?

@yo' Usually the FSII are not added in case of large aircraft having fuel heating systems, though they can be added (for small aircraft, especially) while fuelling.

@yo' At some airports the FSI is premixed, while at others it can be mixed while fueling so is optional.

Ok thanks. That's interesting (for me as a layman, I mean...)

In regions and weather conditions where this is likely to be a problem, other fuels are used instead of Jet-A.
For example, Jet-B has a freezing point of -60C and is used during the winter in some parts of Russia and other very cold places.
For information about fuel freezing in flight, see this previous question/answer.

authors: Адилжан Бектемиров


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How are in-air refuelling craft grounded to each other?
This answer mentions that before refueling equipment is connected the F-16 and the refueling vehicle must be connected to each other. This does not seem to be the case for in-air refueling.
Why is the above-mentioned restriction not practiced for in-air refueling? Is there no chance of electrical voltage potential between two flying aircraft? Are the fuel lines purged before connecting in flight? Are fuel explosions not a big problem while the craft are flying? Are in-air safety considerations lower than on-ground safety considerations?

I would have to imagine that the precautions are required to avoid vapors from igniting. If you're refueling in-air, the vapors will probably dissipate with the 100+kt wind!

The metal of the tanker's boom (or drogue) contacts the metal of the receiver's receptacle (or probe) first, and that contact is maintained, so that would serve to ground the two aircraft to each other.
It's a good question, but I've never heard of any fuel explosions due to static buildup during air-to-air refueling, so evidently the systems in place alleviate the risks.  Aerial refueling is pretty routine these days, so in relative terms it would seem to be fairly safe.
Purging the fuel lines is a relative term; you could remove the liquid fuel from the lines, but you'd still have vapors, and those are more flammable than the liquid jet fuel itself.  Maybe somebody who flew (or flies) tankers can discuss how far down the boom the fuel goes BEFORE everything is connected and they start pumping; as a receiver I just remember that the fuel flowed into the manifold and then into the tanks.  We didn't do anything special to clear the manifold or fuel lines on our end.
There ARE some inherent risks when refueling between two aircraft flying in close formation that are accepted (i.e. the risk level is okay for the military mission, but nobody does it for commercial airliners), but I don't think this is one of them.

By purging I was thinking of removing the vapors with i.e. nitrogen going down the pipe.

@dotancohen Never heard of that being used in tankers or receivers, and given the safety record of modern aerial refueling, I'm guessing it's probably unnecessary.  Airliners (the more recently built ones, at least) do have a system that puts nitrogen-enriched air (not pure nitrogen, but more N2 and less O2 than ambient) into the fuel tanks to reduce the risk of a "TWA 800" (fuel tank goes boom) scenario.  Not required on all aircraft at this point, though.


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Under what conditions are passengers required to take precautionary measures during refueling?
I was on a recent flight with a fuel emergency, during which the passengers were required to take precautionary measures. Under what circumstances are the precautionary measures mentioned below required?
The plane was diverted to a small regional airport apparently unequipped to service the Airbus we were flying in. Though the fuel truck had arrived at the plane and cones had been placed around the aircraft, we waited quite some time for the fire brigade to arrive before the fuel could be pumped. During this time and during the refueling operation, all the doors were opened (in the rain) and the passengers were required to be in our seats with no electronic equipment. Additionally, seatbelts were required to be unbelted.
The fuel emergency occurred while we were circled the Atlantic for 45 minutes waiting for the weather to clear up at Miami International Airport. When the alternate airport shut down as well (Fort Lauderdale International Airport) we were diverted to a small airport in West Palm Beach. From what I was told by a crew member after we had landed, we did not have enough fuel reserve to safely make it to Orlando.
Myself and other pasengers were under the impression that the in-the-seats-with-no-belts restriction is standard for refueling with passengers on board, but in the comments of another question I discovered that this is not the case. What are some likely reasons that these precautions were taken?
The incident occurred on 2014-05-15 on Iberia 6123 leaving Madrid at 11:50 with a scheduled arrival at Miami at 15:20. I'm pretty sure the plane was an A330 but it might have been an A320.

found the flight on flight aware: http://www.flightradar24.com/data/flights/ib6123/#35633f6

Thank you! I see that the WPB airport that we landed at is listed as "International", surely they would be equipped to handle an A330? Note also that there was an additional flight from WPB to MIA that I don't see mentioned there.

@dotancohen The PBI airport sees mostly smaller aircraft. The Bahamas are a short hop away, which is probably the reason for the "International" in the name. Regarding the PBI-MIA flight, although sometimes they keep the original flight number, it appears this time they did not. It's harder to track down a flight more than a couple weeks ago without knowing the flight number.

I see, thanks. I notice that there is an A320 landing right now so apparently the airport does handle them! I thought that the A320 and A330 were similar craft (at least the cockpits look the same from the inside). I suppose that "similar" doesn't mean that the ground equipment used to service them is the same, though.

I always thought the passangers had to exit the plane before it could be refueled.

I'm the one who originally commented that these requirements seemed odd. From the answer below and poking around Google, it does seem that this is at least a not-unheard-of requirement. However, I could swear that I've been on board several airliners and witnessed pressure refueling in progress out the window and didn't have any special instructions or precautions taken by the cabin crew. Weird!

Did you happen to notice HOW the plane was being fueled? The standard way at large airports is a large hose connecting to a port under the wing. Internal plumbing delivers fuel to the aircraft's tanks, the process is fully sealed so leaks and fumes are rare. If the airport doesn't have the right connectors then they do it the old-fashioned way - pop the caps off and pump it in like your car. Spills and fumes are very likely.

@paul: I couldn't see from my seat!

@dotancohen, A320 is a narrow-body with a MTOW about 200t, A330 is a wide-body with a MTOW about 500t.... not very similar.

It was in fact an A330. There is something telling about my priorities when the cockpit was more memorable than the seating arrangement!

Doc 965, CAT.OP.MPA.195(b), regarding Refuelling/defuelling with passengers embarking, on board or disembarking, says:

For all other types of fuel, necessary precautions shall be taken and the aircraft shall be properly manned by qualified personnel ready to initiate and direct an evacuation of the aircraft by the most practical and expeditious means
  available.

So no-belts restriction is one of the elements that ensures "expeditious evacuation". The same goes for keeping the doors open and keeping the passengers in their seats.
Although the legal requirement is not that specific, probably the detailed procedure (with all the elements you described and others that were not visible for the passengers) can be found in the company Operations' Manual.

The seat belts undone make sense, but I don't see it for the door. Unless all of them had stairs, closed and armed so the slides would deploy on opening them sound better to me. Or does A330 allow easy deploying of the slides from open door?

I can't say for sure if the doors have stairs during such a procedure but that would seem to be the easiest way to ensure evacuation. The only time I saw this happening I was a passenger, and we were boarding a 777 during refuelling, so the stairs were obviously there :).  I will find out more from people actually flying commercial planes and return here with the information

All door? I would expect one open door with airstairs and the remaining door closed with slides armed.

(A320 has 4 doors and A330 seems to have 8)

@JanHudec: You were right. Not only it doesn't make sense to keep all doors open, it might actually be against the procedures. In most companies the doors on the refuelling side should be closed. On the other side any combination of open with stairs/closed and slides armed is ok (again, depending on the company)

Every operator has their own procedures, precautions, and "the way they do things."  Sometimes these may differ based on who is doing the refueling -- the airline's own personnel or contractors, as opposed to somebody who is less of a "known quantity".
What is described above sounds very different from what generally happens when US airlines refuel.  To the extent that you want to have emergency evacuation capability in case something goes wrong during the fueling, keeping doors that aren't connected to a jetbridge or airstairs open is UTTERLY counterintuitive.  The door OPEN means you can't deploy the slide quickly.  Closed with the slide armed is generally preferred, IF you don't have a jetway/airstairs connected to facilitate evacuation.  With routine refueling between flights, US carriers typically consider the jetbridge by itself adequate, and the doors aren't armed.  (This is to avert the risk of a catering truck pulling up to an armed door -- opening a door & getting a face full of evacuation slide is a bad day for everybody involved!)
The "seatbelts off" instruction seems pretty gratuitous as well -- the time it takes to release a seatbelt is inconsequential compared to the time it takes a bunch of people to exit an airplane -- even with great urgency.
But, every operator has "the way they do things."

Thank you Ralph. It did not even occur to me that the slide could not be activated without reclosing the door and then arming it. Perhaps the doors were open for ventilation, with the engines shut down?


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How common is it for airlines to fly unauthorised flight paths?
With the revelation that flight QZ8501 did not have permission to fly the route on the day it crashed, I'm wondering how common an occurrence this is.
Is it something that all airlines do every now and then and is actually not a serious thing? Just seems it would hardly be an oversight - as you'd think since they have to advertise tickets, they would've planned to take that route on that day months in advance. It should be a fairly straight forward thing to check whether they were authorised to fly then.

If you look at the weather radar picture here, one reason could be to circumvent a tropical storm. This cannot be planned days in advance and is one of the reasons why airliners take plenty of reserve fuel for their trips. It might not be common, but normal.

It is totally unclear from the question or from the media I've read what was unauthorised about the flight or what kind of authorisation was missing. So it is impossible to answer how common that is.

Flightplans are plans. They can be, and often are, amended at any time, either at the request of the pilot or by ATC.

The permission you refer to was regarding route rights.  It flew that route often, just not on that day.  The permit was for Monday, Tuesday, Thursday and Saturday,.  So far, this is supposition since it has been reported only in newspapers whose track record of reporting aviation matters accurately is very poor.

looking at this image on Wikipedia it looks like the plane lost contact when it entered the red (colder) zone, just twelve minutes after asking permission to turn left.

@Simon - Hi, yes this is what I'm referring to. Regardless of whether the AirAsia did fly without this permit or not, I think it's still a valid question whether airlines do indeed 1) fly a route without any permit 2) fly a route on days which is not included in the permit

Having read the Wiki article, it states that Air Asia doesn't have permission to fly that route on Sunday, and I take that to be the question - How often do airlines fly routes on days they don't have permission to? At least, that's the question I would ask.


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What is the standard way for the ATC to ask a pilot to repeat him/herself?
There is a lot of information on how the pilot should ask the ATC to repeat a statement, but is there a standard way for the ATC to ask the pilot to repeat a readback, or any new information that the pilot provides?

If they are busy on another frequency or on a landline, I will frequently get, “Aircraft calling Center, go ahead”. I also hear, and have occasionally gotten, “N123 Say again intentions”. It’s their way of saying that what you asked for doesn’t make sense and giving you a chance to clarify. But mostly they just didn’t catch what you said and need you to repeat it.

@JScarry The excuse of the controller being on the landline is so common that I've heard pilots jokingly use it, too. "X Center, Say Again. I was on the landline." :) Whether ATC is amused is another question...

Yes, and it is exactly the same: Say again.

Station calling Georgetown Ground say again your callsign
Lufthansa 123 say again speed
Scandinavian 123 say again all after 'good morning'

Or simply

United 345 say again

Don't know why i thought it would be different actually, given the structured nature of communication. Thanks!

@Priyank It's not a dumb question, since some ATC communication is asymetrical.  For example, in the US, a pilot says "departure" ("ready for departure"), but the controller says "take off" ("cleared for takeoff")

@WayneConrad That's universal and applies to both controllers and pilots. You only say "takeoff" when giving or reading back an actual takeoff clearance. In all other cases, you refer to the takeoff as "departure"

In the pilot/controller glossary:
SAY AGAIN

Definition: Used to request a repeat of the last transmission. Usually specifies a transmission or portion thereof not understood or
  received; e.g., "Say again all after ABRAM VOR."

Notice this glossary is specifically "pilot/controller" they use the same terminology.  There is no separate "controller-only" and "pilot-only" glossary of ATC terminology.

I'm told that the background is that "repeat" was used, historically, to mean "repeat the last action I commanded", and specifically, in the days of cannon fire under command of a flagship via signal flags, to mean "shoot another volley with the same aiming as the last one". Thus a completely different way was needed to ask for a re-statement of the last message. This carried on into naval and aviation radio usage.

Controllers routinely monitor multiple frequencies and shout lines.  Typically, the air-to-ground communications (pilots) is directly in a controllers ear, while landline, or shout-line communications (other controllers), are in the overhead speaker.  When a controller answers the landline, the communication is reversed.  The landline (controller) is now in the controller's earpiece, while the air-to-ground communication (pilot) is in the loud speaker.
We give the other controller our undivided attention to ensure proper coordination.  Remember, we're often coordinating lots of numbers.  We hear the pilot calling, however we typically don't retain the details clearly.  Hence, we'll say we were on a landline, please say again.

"'hear' is what is happening" <-- ha! Great typo! Feel free to click [edit] to fix that up. :):)

Welcome to Aviation.SE!  I’ve edited your question a bit.  If I removed any important details, please feel free to roll back to the previous version or edit them back in.

This is good and valid information, but does not actually answer the question


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What are the security measures preventing someone from chartering a 747 and using it to commit a terrorist attack?
While most travelers are well familiar with the security measures imposed by conventional airlines, it is unclear how they apply to private aircraft.
Is it possible for a private individual to charter a 747 for private uses? And if so, are there any security measures in place to prevent said private individual from misusing the aircraft?
(hopefully this question won't get me on the no-fly list) :)

Why exactly do you need to know?

The question is whether something is stopping an evil person from chartering and flying a 747 (or a different plane of similar size).

I was mostly teasing you.  The odds that a terrorist would be investigating the countermeasures in order to defeat them are, I would think, low.

@Malvolio I would think just the opposite.  I'd guess that they are constantly testing to see what they can and cannot get away with so that they can make plans for the future.

Why charter when you can hijack?

@IanRingrose because it might be easier.

Because it's pretty much impossible to hijack an airplane after 9/11. The cockpit doors are reinforced and the passengers are more than willing to stop a potential hijacker.

The main defense is that chartering a large airplane is expensive.

Yes!  You can charter large aircraft like the 747.  A lot of airlines offer this service for large groups (sports teams come to mind), and there are also operators of the Boeing Business Jet that were created just for private and charter uses.
The TSA requires aircraft like this to adopt a security program called the Twelve-Five Standard Security Program (or TFSSP for short).
49 CFR Part 1544 contains the regulations which cover this.
Most of the security program falls under non-disclosure rules that prevent anyone with knowledge of the program from talking about it to people that don't have an operational need to know (and have also signed a non-disclosure).  This helps to prevent people that would try to bypass the security measures in place from even knowing what to look for.
The regulations do specify the following though:

Each operator of an aircraft that weighs more than 12,500 lbs. that is used for scheduled or charter operations must adopt the program.
The program must be approved by the TSA.
The program must contain the procedures and description of the facilities and equipment used regarding:

the acceptance and screening of individuals and their accessible property, including, if applicable, the carriage weapons as part of State-required emergency equipment,
the acceptance and screening of checked baggage,
the acceptance and screening of cargo,
the screening of individuals and property,
the use of metal detection devices,
regarding the use of x-ray systems,
the use of explosives detection systems,
the responsibilities of security coordinators,
the requirements for law enforcement personnel,
carriage of accessible weapons,
carriage of prisoners under the control of armed law enforcement officers,
transportation of Federal Air Marshals,
aircraft and facilities control function,
the specific locations where the air carrier has entered into an exclusive area agreement,
fingerprint-based criminal history records checks,
personnel identification systems,
training,
an aviation security contingency plan,
bomb and air piracy threats,
flight deck privileges, and
the Aircraft Operator Implementation Plan (AOIP).

As you can see, there is quite a bit that goes into the TFSSP.  Two of the biggest things though are the requirement for all flight crew members to undergo a fingerprint based criminal background check and that access to the flight deck must be restricted.
For more details, refer to the actual regulations which cover this in more depth.
NBAA also has a page about the TFSSP.

I did not know that there was no many way someone was allowed to take a gun on a aircraft!    This list does not make me feel safer.

@IanRingrose It isn't for the average person.  Just like on the airlines, a gun can not be accessible in flight, must be unloaded, etc. Federal air marshals, certain law enforcement officers, etc. can have a gun on an airliner as well.

@Lnafziger, But it makes it possible for the person sitting next to the law enforcement officer to get the gun and use it on me.   If the gun was not there I would be safer.

@IanRingrose That was one of the arguments against allowing pilots to carry guns as well, but at least in this case it is a trained law enforcement officer. It isn't impossible, but it is less likely since they are trained to protect it and keep it from falling into the wrong hands. To be honest though, after what happened during the September 11th attacks, I can't see passengers letting even a person with a gun into the cockpit of an airplane. A chance of being shot is better than certain death when they crash. Most of these are rare though and won't be on every flight.

It is amazing that since the September 11th attacks and passengers no longer being willing to just sit back and let hijackers get on with it, that we have had very few hijackers.   Yet we were all taught to just to what a hijacker told us to.   A pilot having a gun behind a locked door at least knows when someone is trying to attacked them, and could be justified is shooting anyone that opens the door without getting clearance first.

@IanRingrose If the passengers on the 9/11 flights were allowed guns, there wouldn't have been any planes flying into buildings. Lnafziger - All those regulations are in the end just pieces of paper that have to be signed. If someone wanted to fly a plane into something, they easily could.

@Illidanek If they are already flying the airplane, then there isn't much to stop them.  I wouldn't say that a passenger could "easily" get into a 747 cockpit and fly it into something though.

Lnafziger - no, of course. I suspect it's not entirely difficult to blow up the plane, but hijacking is in fact very hard.

I agree, there wouldn't be much to stop a terrorist crew. Except I think in this post-9/11 era, a private airplane not communicating with ATC and flying off-course towards a populated area would quickly be met with escorts and possibly shot down.

This is an American-centric answer. What about the rest of the world? What's to stop someone in Europe chartering a private plane and using it to commit a terrorist act? Even flying to the USA to do so, for example.

@ChrisMelville Well....  This question is tagged TSA, so it was purposely American-centric.  If you have a question about a different part of the world, then I would suggest asking a new question!

I haven't spoken to any terrorists, so the only source I have for this is Bruce Schneier, but apparently, they're hard to pull off. Objectively, this makes sense; if they were easy, there would be more of them.
For the sake of not referencing any actual living or dead person, let's call our supposed terrorist "Hans Gruber". Hans has enough money to charter a 747 and the desire to terrorize someone. How does he go about it?

If he charters the plane himself, takes control of it, and then crashes it into something (the Nakatomi tower, perhaps), and succeeds, then he dies. Surprisingly, I don't know of many people who are (a) willing to commit terror, (b) willing to personally die for their cause, and (c) multimillionaires.
Okay, what if Hans puts up the money but hires someone else to fly?  Well, money comes from somewhere and charter operators tend to remember you when your passenger hijacks and crashes your 747. Suddenly now you have the authorities after you. Plus, there's no guarantee that the person you hired is going to get the job done.
Okay, but what if Hans is the president of a group of shell companies that can funnel money here and there and disguise the source? Sorry, no good there either. The more people who know about your plan, the more likely the NSA knows about it. At some point (and I would guess that point is not very large) more people in the know results in diminishing returns.
Okay, but what about an opponent who has lots of money, enough willing people to pull off the plan, resources to get them in place and provide the proper training, all while disguising the source? A vast conspiracy? Well, yes, those exist. They're called nations, and other nations go to war with them over things like that. But at that point, it's not really terrorism anymore, is it?

Loved point 4.... harsh reality but true

2b. "hans" puts the money and brainwashes someone else to fly it.

Objectively, if terrorist attacks were easier to pull of than they actually are, there would be more of them. That doesn't mean they're really hard though.

Point 1 is not 100% valid. Although I don't know of anyone who is willing to commit terror and personally die for their cause; these people exist as was evident in the September 11 attacks in USA, 7 July bombings in London, etc. Plus you don't need to be a multimillionaire. Charter rates are around $40,000 per hour.

lol, my favorite part of this answer is when you point out that nations are really just vast conspiracies.  Good one there :).  In reality the best points are 1(a)(b)(c).  Supremely irrational people with resources are, thankfully, few and far between.

@MattWilko True, though it tends not to be the organizers who go on the suicide missions.

IIRC, the Army was letting people charter a C-130 for a very reasonable sum, $3500 an hour or so.  Their pilots, of course.  You don't have to be particularly rich to pony up the $10 or $20 thousand it would cost to charter a wide-body for a short run.  Of course, the problem of seizing the plane and then using it remains.

@Malvolio, (un)fortunately, you don't need an extreme amount of skill and training to crash a plane. The problem of seizing it remains. :)

Yeah, I guess "crashing" is the plane's default behavior.  And if you chartered the plane, the pilot works for you and you can just say, "Uh, Captain, can I get your opinion?  Does this handkerchief smell like chloroform to you?"

@Malvolio I would think that seizing a military aircraft, with military pilots, load masters, etc. would be considerably more difficult than other chartered aircraft, lol.  There's also the part where they might think it's suspicious when you charter a C-130 and show up without any cargo and only want a short flight  (let's see, here's the $875 for 15 minutes....  Do you have change?)

This is not really an argument that it's hard, but rather that it's hard to get away with it.

For point 2, what about when Hans has a pilot willing to die for his cause and hands him a bag of cash. The pilot then contacts the charter operator and there is no Hans for them to remember.  This is how a lot of the terrorist activity that I have read about in the news works....

More likely, Hans kidnaps the pilot's family, and tells him that he will torture and/or murder his wife and children, if the pilot doesn't crash the plane.  (Sorry, or husband and children - the pilot might be a woman)

I honestly don't see why this answer is so heavily upvoted - it doesn't address the question in the slightest. Also, 9/11 was funded by millionaires, i.e. Bin Laden.

@DannyBeckett - When I started typing, I am pretty sure the question didn't contain the words "security measures", it just said "What's preventing someone from chartering a 747 and using it to commit a terrorist attack."  (Either that, or it did and I just misread it because I had just woken up.)

Hey, no worries - maybe the OP edited in the grace period - that'd make sense! Perhaps you could edit your answer a little :)


text:
Can I get a photo with the pilots?
Could I, as a passenger, get a picture with the pilots in the cockpit before takeoff/after landing? I'm asking specifically about any rules that would deny a passenger into the cockpit. I know this is too broad and subject to change from airline to airline, but generally would I be allowed? What about domestic vs international flights?

What country is this for?  I know the TSA has some pretty strict rules, not sure about other countries though.

Let's use US as an example

Not sure of the answer, but it might depend on the particular pilot you get.  And your ethnicity/ general appearance.

The TSA has no rules nor jurisdiction over the cockpit

Last December the pilot invited my then 4-year-old daughter to come into the cockpit and use the PA system to wish the passengers a Merry Christmas

I don't know about the situation specifically in the US, but in my - mostly European - experience crew are happy to welcome you to their office after flight.
When I feel the need to see the cockpit or discuss things with the pilots I try to ask the flightcrew before take-off if I can meet them after the flight. Sometimes I explain my interests to the cabin crew and often they will arrange the visit. In a few cases I just asked while deboarding.
Depending on the aircraft type and airport situation you have to think about the logistics a bit. In single aisle aircraft, if you start a discussion with crew while deboarding, you are blocking the queue. This gives the feeling that it is an inconvenient moment and reduces your chances. So if you ask at the last possible moment, make sure most people already exited before you.
However, if passengers are transported to the terminal by bus, you will be holding the bus since you are the last to get off. This reduces the time you can spend in cockpit drastically. If you expect to be bussed to the terminal, ask cabin crew before the descent and don't wait until everybody has exited.

Or you can get to ride a mini bus with the crew. This happened to me on an Air China flight when I was searching for a stolen item in the cabin.

If you were going to try to do something like that, it would depend greatly on the crew.  You may be limited to getting the picture taken in the doorway to the cockpit, and for the sake of security, I would suggest after landing (generally when the pilots are doing their, "Bye, thanks for flying" thing as people deplane).
You would probably have to coordiante with the flightcrew, and make sure you are not impeeding progress of other passengers. Furthermore, if you do it, others may want to do the same, so again, that may factor into the decision.
If you are going to try, I would suggest doing it on an airline that has a reputation for being more "fun" (like Southwest).  Also, many airlines have a sort of "public affairs" line that you can call and try to pre-coordiante such activity.

Most pilots I know are quite happy to welcome a visitor to the cockpit between flights for a quick photo or a little "show & tell."  Sometimes things are just too busy, but mostly it's a pleasure to do so.

As you have said, cockpit entry rules vary depending on the country and even airline.
In the US visitors are fobidden from being in the cockpit when the aircraft is in flight, but to my knowledge it is still legally permissible when parked at the gate. Airlines may have their own rules surrounding entry though.
I haven't tried in the US, but elsewhere I find that heading to the cockpit as really depends on the crew members and workload. Before or after a short flight with a quick turnaround the pilots may rightly say they are too busy. After a long international flight they may just want to get to their hotel!
I suggest that in cruise, when the FAs don't look too busy, just politely ask one of them if it would be possible to visit after the flight - explain that you're an enthusiast, etc. It'll help if you've got a kid with you! Hopefully the FA will ask the flight crew and you'll get a positive answer. If you're really lucky (and in the right part of the world) you might even get an in-flight invite!
Remember that pilots are real people. Most of them quite enjoy showing off their office to excited people when their circumstances permit.


text:
Why do some airlines operate fleets so diversified?
When you have a look at the fleets from big airlines like United, Delta, Lufthansa, BA, AF, etc. it seems that they have almost every model Boeing and Airbus released in their fleet. And my question is, why?
What I assume is that such a big variety would increase the costs for maintenance, training, etc.
Thus, for example, why use the A330 as well as the 777? Why operate both the A320 and 737?
For me as a passenger it is comfortable because I like to test and review different aircraft; but for these airlines' Operations divisions it must be horrible!
I know there are probably reasons to do so otherwise they would not have this variety. What are these reasons?

There are two philosophies for fleet selection:

One type / model or one manufacturer approach
Multi manufacturer divide and conquer approach

First of all, a single model fleet is only possible for airlines that operate on sectors of comparable length. You can't do intercontinental and regional flights with the same aircraft model efficiently.
As you observe, there are obviously costs associated with flying different types of aircraft. A single type fleet like for example Ryanair (B738), Southwest (B73*) and Easyjet (A320 family) operate, means the cost of maintenance, training, etc can be kept low. Adding a few aircraft of another type to the fleet would mean additional tools in stock, diversified training and a separate flight crew group and lots of other cost factors.
Aircraft manufacturers know that too. So if you need another 10 - 20 aircraft, your standard supplier will ask a high price for this fleet extension, knowing that the competition is never able to put in a compelling offer when all additional cost are taken into account. Only if the batch size is large enough the competitor might be able to offset the additional costs with a unit discount.
That is where the mixed fleet operator has an advantage. Since the additional overhead costs of a couple extra aircraft from type A or B are minimal, they have a better position while negotiating with the manufacturers.
Single manufacturer airlines usually order aircraft in large numbers, otherwise they would not get a proper deal; multi type airlines can order aircraft in lower numbers.

And then there are politics. Sometimes a national airline will be subjected to political pressure to buy from a certain manufacturer. This is especially the case for airlines operating in the international / intercontinental market. The extra fleet cost incurred will be (partially) offset by deals in another field.

Another advantage to a homogenous fleet is that you can sometimes buy your airframes on the secondary market, especially when you operate a very common type of aircraft. Southwest has always been a big proponent of this, often acquiring used 737s when the fleet needs expanding.

@voretaq7 that secondary market is as accessible to diversified airlines isn't it or am I missing a point? In fact, getting models from the second hand market breaks the single model concept a bit since cockpits are somewhat customized to the first owner. You end up with different types of FMSes etc.

EasyJet, technically, don't have a single model fleet: they have A319s and A320s (and A320neos on order).

@gsnedders The A318, A319, A320 and A321 are considered to be the same model (A320 family) in the way the -600 , -700, -800 and -900 are all considered 737NG models. Similar / same parts, same type rating, etc. But if you want to make the distinction you should start with Southwest, they operate the classic 737 alongside the 737NG. Only Ryanair adheres strictly to the B738 since a couple of years.

Thank you for your answer. Your example with fleet extension makes sense except one point, if the competition is never going to put in a compelling offer, why has JAL ordered Airbus aircraft for the very first time in their history?
I thought on political pressure but cannot image why this might be the reason.

@jp_ Several reasons I can think of: 1. Given the size of the order (31 large aircraft) Airbus had room to reduce total order price into the attractive range. 2) Airbus really wanted to get into the Japanese market 3) Boeing has been sole supplier to Japan for many year, probably complacent. 4) The 787 teething problems were still fresh in mind. Fear to rely on a single vendor 5) The A350 has no real competitor yet, it is available sooner then the B777X. 6) Airbus uses relatively few parts from Japan in their products -> business opportunities for Japan / politics.

@jp_ unsurprisingly Airbus announced several contracts with Japanese suppliers  after the JAL A350 deal.

@DeltaLima Ah, I've always considered them different models of the same "series". Differences in terminology, I guess…

@DeltaLima absolutely - anyone buying a plane from a common and long-lived production family has the option of the secondary market. It's also not without shortcomings as you've pointed out (I'm not sure how Southwest handled cockpit standardization starting up their fleet).

@gsnedders After giving it another thought, I've changed model to type in the text. Model is often used in the way I described it earlier, in the ICAO Make/Model/Series taxonomy. However in certification taxonomy, the term type is used. The A318/A319/A320/A321 have a shared type certificate. In that taxonomy the A319-114 is a model which is part of the A319-100 series.

@DeltaLima I thought manufacturers these days were reluctant to do custom cockpits for different customers?

@DavidRicherby Of course the manufacturer prefers no customization at all ("you can have any color as long as it's black") and the airlines want to be able to choose their own equipment (not limited to the cockpit) to prevent vendor lock-in. If you need an upgrade to some equipment part (e.g. the GPS) during the lifetime of the aircraft and you cannot switch to another GPS manufacturer, you'll pay a lot for a simple firmware upgrade.

One reason that I have not seen addressed in any of the extant terrific answers is acquisitions. Many times over the past few decades airlines have been sold, merged, or folded. Each time, the airlines' aircraft go to new owners who may or may not already operate acquired aircraft models at the time.
As the airlines' owners change, there is churn in which airlines operate which equipment.

Some amount of diversity is good for business.
From time to time, an airworthiness directive might force all planes of a particular model to be grounded.  This has happened for the 787, for example, due to concerns over battery safety.  Even with more established models, urgent safety issues may surface.  A high-profile example would be the Airbus A330 and A340 pitot tubes, after the Air France Flight 447 accident.  Grounding the entire airline's fleet would be an unacceptable business risk.
Having a fleet of planes from more than one manufacturer could result in better deals during purchase negotiations.

The A380 would be another example of an aircraft that at least some airlines grounded for a while in the aftermath of the uncontained engine failure in QF32.

This answer definitely aged well given the 737 MAX fiasco.

Airlines usually try to avoid having more than one type for each size exactly because it makes maintenance more complicated and expensive. Low-cost carriers go even further and operate only one type altogether.
Airlines that have diverse fleet usually have acquired them by merging with other companies that had different fleets. Note that all the companies you mention each swallowed several other airlines recently enough to still have those aircraft in active service.

Most of the primary reasons are already covered here, but another reason can simply be availability. This is part of the reason cited for Delta's recent choice to order A350s and A330neos rather than more 787s. While Delta already has 18 787-8s on order, the lack of availability of additional order slots in the time frame that Delta needed to replace its aging 747s and 767s was, according to Boeing, part of the reason that Delta chose to order A350s and A330neos rather than adding to its existing 787 orders. Additionally, the 777X was also not going to be ready to deliver in the time frame that Delta needed.

“This was a long and highly competitive campaign,” said Doug Alder, a Boeing spokesman. “Boeing competed for the order with the 787-9, but we did not have enough 787 positions available in the time frame that met Delta’s requirement.”

And then there's airlines that have fleets of aircraft inherited from other airlines they took over/merged with.
And there's the airlines in the process of replacing their fleet with something new.
Airlines that are expanding and decide to buy new aircraft of a more modern type and keep the old ones for now because it makes no economic sense to scrap or sell them.

The reasons are plenty, even if the airline could in theory use one single type for its entire route network and use it efficiently (which use pattern would seriously limit the scope of an airline's operations, you'd not be able to have for example flights within Europe and flights from Europe to the USA and Japan).

Your last paragraph is irrelevant to the question. The question doesn't ask why airlines generally don't fly a single plane type (e.g., only B737s and nothing else). It asks why there are many airlines that have directly competing planes in their fleets, e.g., the same airline owning both A320s and B737s.

@DavidRicherby Judging by your comments, you seem to be quite knowledgeable on the subject. Care to share an answer?

@AssortedTrailmix Thanks but the existing answers already cover everything I know.

Consider sometimes that an airline doesn't own these aircraft - they may be leasing them for a short term.
Aircraft leasing comes in two flavors - ACMI "wet lease" where all the airline is doing is slapping their logo on the aircraft, everything else Aircraft, Crew, Maintenance Insurance is included; and then a "dry lease" where only the aircraft is leased.
This is normally done during busy periods (for example during the annual Hajj season) many airlines will operate a type different than their normal operations.  The 747 is a favorite for leasing during this period.
So although you may be seeing the aircraft being flown by your favorite airline, the leasing cost may offset the profit for that particular route.

An airliner (a large one as you mentioned) tend to get airplanes of different manufacturers for several reasons. Some are:
Destinations
An airline's airplanes fly to destinations of varying distances and traffic loads from its hub. Operating different types of aircraft according to the needs of that particular route would drive the need of different types of airplanes.
Deals and Incentives
Airplane manufacturers may offer different deals to an airline at different times. Such business decisions are always made to a company's best interests.
Hiring and Training
Airlines tend to hire flight crew with different ratings so that will have a wider experience and proficiency.

This mostly doesn't answer the question. Sure, airlines need planes with different ranges and capacities, so it makes perfect sense for an airline to operate, say, Boeing 777s and Airbus A320s. But the question is asking about why an airline would fly directly competing planes, e.g., Boeing 737 and Airbus A320. And why is it an advantage for an airline to have both Boeing- and Airbus-trained pilots? That's a consequence of having a mixed fleet but it doesn't seem to have any direct advantages on its own.


text:
What does the number of white bars in runway's threshold mean?
At the beginning of each runway there are some white bars.
Does number of bars in threshold mean anything?

For runways built, refurbished or repainted after January 2008, the number of bars in the Threshold Markings indicates the width of the runway, as is described in Section 3 of the AIM. Older runways may still use an outdated scheme.

Runway threshold markings come in two configurations. They either consist of eight longitudinal stripes of uniform dimensions disposed symmetrically about the runway centerline, as shown in FIG 2-3-1, or the number of stripes is related to the runway width as indicated in TBL 2-3-2. A threshold marking helps identify the beginning of the runway that is available for landing. In some instances the landing threshold may be relocated or displaced.

(source: faa.gov)

Runway Width   | Number of Stripes
==================================
60 feet (18 m) | 4
75 feet (23 m) | 6
100 feet (30 m)| 8
150 feet (45 m)| 12
200 feet (60 m)| 16

With regard to how to tell the difference between Configuration A and B, Advisory AC 150/5340-1J goes into significantly more detail. It states that

The threshold markings may have either of the characteristics in (1) or (2) below, but after January 1, 2008, only those characteristics in (2) will be acceptable.

The details:

Configuration A. The runway threshold marking consists of eight longitudinal stripes of uniform dimensions spaced symmetrically about the runway centerline as shown in Figure 1. The stripes are 150 feet (45 m) long and 12 feet (3.6 m) wide and spaced 3 feet (1 m) apart, except for the center space, which is 16 feet (4.8 m). For runways less than 150 feet (45 m) in width, the length of the markings is not changed, but the width of the markings, spaces between markings and distance of markings from the runway edge are changed proportionally. For runways greater than 150 feet (45m) in width, the width of the markings and spaces between the markings may be increased proportionally or additional stripes may be added to both sides.
The rule for Configuration A markings requires scaling the width of the stripes and their spacings (and the outboard spacings from the runway edges) proportionally, keeping the length fixed. For a runway 150 feet wide the Configuration A stripes are 12 ft wide with 3 ft spacings (and an 8 ft spacing from the inboard stripe to the centreline, leaving 10 ft between the outboard and runway edge). The Configuration B stripes are given a fixed width of 5.75 ft with spacings of 5.75 ft also. If a Configuration A scheme were used on a narrow runway such that the stripes were equal to the width of the Configuration B stripes, the runway width would need to be about 72 feet (12 ft stripe width for a 150 ft runway width scaled to 5.75 ft for 71.875 ft). For a 72 ft wide runway bearing the Configuration A scheme the spacing between stripes would be reduced to slightly less than 1.5 ft. The difference between the schemes therefore ought to be apparent.

and

Configuration B. For this configuration, the number of stripes required is related to the runway width as indicated in Table 2. The stripes are 150 feet (45 m) long and 5.75 feet (1.75 m) wide and spaced 5.75 feet (1.75 m) apart except the center space is 11.5 feet (3.5 m) as shown in Figure 1. The outboard edges of the outboard stripes extend laterally to within 10 feet (3 m) of the edge of a runway or to a distance of 92 feet (27 m) on either side of a runway centerline, whichever results in the smaller lateral distance.

This is further borne out by a May 2004 advisory (CERTALERT 04-04) which states:

The purpose of this CERTALERT is to remind Airport Operators, Airport Certification Safety Inspectors, and Program Managers that paragraph 9(d) states the threshold markings may have either the characteristics in (1) Configuration A or (2) Configuration B, but after January 1, 2008, only those characteristics in (2) Configuration B will be acceptable.

I wonder why they chose a number of stripes that can't be eyeballed. It's very hard to look at a bunch of stripes and distinguish 16 of them from 12 without actually counting, though I guess the bigger gap in the centre helps somewhat.

@DavidRicherby - remember you dont have to eyeball 12 from 16 - you have to eyeball 6 from 8 (either side of the centre line). Im pretty sure most people could very quickly distinguish 6 from 8 white stripes.

Interesting bit of downvoting considering there isnt many ways to answer this and with the amount of detail provided.

Can you share an updated picture, as the one you shared is shown as not found :( https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0203_Auto27.png

Is this US-specific (you mention the AIM) or global? OP doesn't seem to specify a location.

at the beginning of the runway are Threshold marking which mark where you can touch down
to quote faa.gov

h. Runway Threshold Markings. Runway threshold markings come in two
  configurations. They either consist of eight longitudinal stripes of
  uniform dimensions disposed symmetrically about the runway centerline,
  as shown in FIG 2-3-1, or the number of stripes is related to the
  runway width as indicated in TBL 2-3-2. A threshold marking helps
  identify the beginning of the runway that is available for landing. In
  some instances the landing threshold may be relocated or displaced.

and table 2-3-2
 Runway Width      Number of Stripes
 60 feet (18 m)    4
 75 feet (23 m)    6
 100 feet (30 m)   8
 150 feet (45 m)   12
 200 feet (60 m)   16

Is this US-specific (you quote the FAA) or global? OP doesn't seem to specify a location.


text:
Can flight data recorders be hacked?
In the event of a plane crash such as MH17, and the flight data recorders (FDR, a.k.a. "black boxes") are potentially in the "wrong hands", is it theoretically possible to:

open the FDR and close it again without leaving traces?
alter or selectively erase data of the FDR?

In other words, can an FDR professionally be tampered with as opposed to purely destroyed for the purpose of hindering an investigation?

Fortunately for MH17 case the CVR and FDR are not very important. There would be nothing unusual before the explosion and the explosion most likely stopped power and so the recording. Important proofs will be radar records observing the missile that USA officials already reported to have (mentioned on AvHerald), debris of the missile itself if the official investigators or OSCE observers manage to find any and traces on debris of the aircraft indicating the explosion happened outside.

I know it's conspiratorial, but what if the plane crashed due to malfunction and both sides are trying to use this unfortunate event as a pretext to other actions?

@jnovacho In that case the examination by investigators will have to find no fregments of anti-aircraft equipment and the remains of the fuselage will have to contain no signs of shrapnel or external damage where it cannot occur, e.g. if one of the engines disintegrated without external influence, the damage on the fuselage will indicate this by the way the debris or engine parts penetrated the structure. I am not sure if chemical traces would be on the fuselage. That's a question for the missile-experts.

Dont know if this is true of current commercial planes, but I saw a recent air disaster documentary that stated that even after removing power the recorders will carry on recording for a few seconds.

@SentryRaven: Or they could just have someone "persuade" the investigators to go along with their version of events.

Yes.  Black boxes aren't designed to be tamper-resistant.
The hard part is not the physical act of tampering with a black box, it's constructing a coherent false narrative of what happened during the time period.  The FDR records 80+ channels of data, which an investigator can correlate against each other, against data in the quick access recorder, against ACARS reports, and against the physical condition of the airplane itself.
For example, say you want to "prove" that an airplane's engines were shut down mid-flight, leading to a crash.  You could modify the FDR's record to show a throttle setting of "closed" and an engine RPM of 0, but the exhaust temperature would still be high, the fuel tank levels would still be dropping, and analysis of damage to the turbine blades would show a mix of axial and circumferential bending around the entire circumference of the blade disk, rather than the pure axial bending on a single side that would be expected if the engines had stopped spinning.

Do you know if there is at least a seal? Has there ever been a reported case where someone has tried to tamper with the FDR?

In the case of MH17, I doubt the FDR would show much of anything that someone would want to hack, (other than possibly its exact position on one side of a border).  It likely showed everything was fine up until the moment of catastrophic failure of multiple systems, all at once.

What would stop someone from reflying a crashed flight in another plane and swapping out the FDRs? Not referring to MH17.

@DannyBeckett: The information recorded by the second FDR wouldn't match the information recorded by external systems such as ATC. It would be extraordinarily difficult to replicate all the conditions of the original flight.

Note, that neither recorder is going to contain anything proving shot-down anyway as they would most likely just abruptly stop. Radar records observing the missile (that USA already officially reported to have) and analysis of the debris can prove a shot-down without the recorders.

But Shirley a device designed to withstand hundreds of G shock, hundreds of degrees fires and submersion in hundreds of meters of water, should to a certain extent be more than a little difficult to disassemble, let alone reassemble.

I see what you did there.

A crash in Peru(?) had its FDR taken by looters- they recovered the box by giving  a reward but the data was too damaged partly by the looter to be useful.